1. Field of Invention
This invention is directed to methods of reducing show through error in duplex printing.
2. Description of Related Art
Duplex printing, i.e. printing in which an image is formed on both sides of a sheet of paper or other image receiving substrate, can be performed using a variety of known systems.
For example, duplex printing can be conducted with a system that forms images on both sides of a sheet at a single transfer station. In some duplex printing systems, after a sheet has received a first image on a first side by passing the sheet through an image transfer station, the sheet is inverted. A second image is then formed on a second side of the sheet by passing the inverted sheet through the same transfer station.
Positioning an image on one side of a sheet in a manner that coincides with the position of the image on the other side of the sheet can be difficult. Registration of a first image on a first side of a sheet with a second image on a second side of a sheet is not always accurate because of one or more registration errors that offset the first image relative to the second image. For example, a page number printed on the bottom-center position of the first side of a two-sided, printed document should align exactly with the page number printed on the reverse side. The offset of the page number on the second side of a sheet with respect to the page number on the first side of the sheet is a registration error that is extremely undesirable, and considered unacceptable in various printing industries.
Registering two images on the front and back sides of a single sheet of paper can be difficult but is essential in industries such as the offset printing industry. In this industry, duplex sheets are sometimes produced having a number of pages, of what will ultimately be a single, multi-page document, aligned on the front and back of a single sheet of paper. To create such a multi-page document, a sheet of paper is printed with multiple images on the front and back side of a single composite sheet. The single composite sheet is subsequently folded and segmented into individual pages. Each of the images on a first side a sheet must therefore be registered with a corresponding image on a second side of the sheet before the sheet may be segmented into individual pages.
Specifically, the first image that appears on the first side of the sheet and the second image that appears on the second side of the sheet are positioned so that identical images printed on both sides of the sheet are coincident with each other. In other words, two identical images printed on both sides of a sheet of paper form mirror images of each other since each image is printed with no apparent offset from the other. Thus, an image on the front side of a sheet would appear to be in perfect or transparent registration with the corresponding image on the back side of the sheet.
To ensure transparent registration, it is essential that the position of the printing substrate be precisely controlled. Active registration systems which sense document position and operate to correct the position of a copy sheet, if necessary, before an image is transferred to the copy sheet are well known. However, even if position is controlled, errors in magnification make achieving such transparent registration difficult. Errors can be attributed to the speed at which an image carrier, such as a photoconductive drum or photoreceptor belt or drum, operates. Magnification errors can also be attributed to the frequency at which a write clock or a pixel clock operates. Another source of magnification errors is the expansion or contraction of paper, coupled with variation in these properties from sheet to sheet. In order to correct such magnification errors, the speed of the photoreceptor belt or drum, or other such device, is adjusted, and the pixel clock frequency is adjusted.
The xe2x80x9cshow throughxe2x80x9d error that occurs when transparent registration is not achieved can be quantified by measuring of the displacement between two points, one on a first side of the sheet and one on a second side of the sheet, that are intended to be equidistant from a common sheet edge. This error is caused, at least in part, by the factors identified above. The portion of the error associated with paper shrinkage is often caused by fusing a printed image on the first side prior to printing of an image on the second side.
In various printing systems, which combine a wide range of paper types with very specific performance specifications, a method of reducing show through error is needed.
This invention provides systems and methods that make margin adjustments to reduce the effects of show-through in duplex printing.
This invention separately provides systems and methods that reduce show through error in duplex printing that compensate for different paper types and sizes.
This invention separately provides systems and methods that reduce show-through in duplex printing resulting from paper shrinkage or growth.
This invention separately provides systems and methods that reduce setup errors that occur when adjusting simplex and duplex magnification errors.
This invention separately provides systems and methods that compensate for errors that result from paper shrinkage caused by fusing during duplex printing.
In various exemplary embodiments of the systems and methods of this invention, show through is reduced by performing registration setup to adjust a pixel clock frequency and/or a photoreceptor belt or drum speed, determining an amount of residual magnification error after performing registration setup, determining margin shifts to reduce the amount of residual magnification error, and applying the margin shifts. In various exemplary embodiments of the systems and methods of this invention, paper shrink effects on registration can be compensated for using determinations made during a typical printer setup. Show through errors can be reduced without using a paper conditioner to pre-shrink or re-wet the paper.
In various exemplary embodiments of the systems and methods of this invention, the relationship between first side image and the second side image is evaluated to determine how the position of the paper and/or the size and arrangement of an image can be manipulated to compensate for the paper shrinkage effects caused by fusing.
In registration systems that use a common edge for simplex and duplex registration, the show through errors become progressively worse as the image moves away from the registration edge. Using information determined and stored in a non-volatile memory of a printing device during an image-on-paper registration setup, determinations can be made to apply a margin shift that results in a significant reduction in the maximum show through for each image.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.